In humans, the blastopore becomes the anus. This is the defining feature of deuterostomes, the broad animal group that includes all vertebrates, and the word “deuterostome” literally means “second mouth” because the mouth forms later as a separate opening. Humans share this developmental pattern with other chordates, starfish, and sea urchins.
What the Blastopore Is
The blastopore is the first opening that forms when a ball of embryonic cells begins to fold inward during a process called gastrulation. Think of pressing your thumb into a soft rubber ball: the dimple that forms is roughly analogous to the blastopore. This infolding creates the earliest version of a digestive cavity and establishes the three fundamental cell layers (ectoderm, mesoderm, and endoderm) that will eventually build every tissue in the body. In humans, gastrulation occurs during the third week after fertilization.
Deuterostomes vs. Protostomes
Animals have historically been sorted into two major groups based on what happens to the blastopore. In deuterostomes, including humans, the blastopore persists through development and becomes the anus. The mouth forms separately by breaking through the outer cell layer at the opposite end of the developing gut. In protostomes, a group that includes insects, mollusks, and worms, the blastopore was traditionally said to become the mouth.
Recent research has complicated this picture somewhat. Detailed cell-tracking experiments show that in many protostomes, the site of gastrulation rarely becomes the mouth directly, even though differential growth shifts tissue toward the oral opening. Some protostomes even form an anus from the gastrulation site, just like deuterostomes do. So the old classification labels are less clean-cut than textbooks once suggested. Still, the core principle holds for humans and other deuterostomes: the blastopore gives rise to the anal end of the digestive tract.
How the Mouth Forms Instead
If the blastopore becomes the anus, where does the mouth come from? In vertebrates, the mouth forms by a breakthrough event. The outer cell layer (ectoderm) and the inner digestive lining (endoderm) come into direct contact in a region near the head end of the embryo where there is no middle cell layer between them. These two layers fuse and then dissolve, creating a perforation that connects the outside world to the developing gut. This structure is called the buccopharyngeal membrane, and its breakdown is what gives you an actual mouth opening.
The sequence involves several steps: the oral ectoderm and foregut endoderm are first specified, they become juxtaposed during gastrulation, the basement membrane between them dissolves, a pit forms (the stomodeum), and finally the membrane perforates. It is a precise, multi-step process, which is why developmental errors in this region can lead to rare facial malformations.
The Blastopore Equivalent in Human Embryos
Human embryos don’t look like the simple spherical embryos of sea urchins or frogs, so the blastopore doesn’t appear as a neat round opening. Instead, the structure considered equivalent to the blastopore is the primitive streak, a narrow groove that forms along the back surface of the embryonic disc during week three. At the front end of the primitive streak sits a small channel called the neurenteric canal, which briefly connects the developing gut cavity with the space that will become the spinal canal. This neurenteric canal is the closest structural match to the classical blastopore seen in amphibians. It remains open for only a few days in the human embryo before closing.
Classical embryology texts describe the primitive streak as arising from a linear fusion of the blastopore’s lips, essentially the same structure stretched into a line rather than remaining a circle. The cloacal membrane at the tail end of the primitive streak marks the boundary that will eventually contribute to the anal opening.
What Goes Wrong When Gastrulation Fails
Because gastrulation sets up the entire body plan, errors during this stage can cause serious congenital conditions. If the primitive streak duplicates, conjoined twins can result. If remnants of the notochord (a structure that forms through the blastopore region) persist in the wrong place, they can develop into chordomas, which are rare tumors of the spine or skull base. An accessory neurenteric canal that fails to close properly can leave behind a neurenteric cyst, a fluid-filled sac trapped between the spinal cord and the gut.
Problems with the tail end of the process, where cells migrating from the primitive streak region build the lower body, can lead to caudal agenesis (incomplete development of the lower spine and pelvis) or, in extreme cases, sirenomelia, where the lower limbs are fused. Currarino syndrome, a condition affecting the sacrum and rectum, has been linked to mutations in a gene involved in this same caudal development.
Signaling That Controls the Process
The molecular machinery that determines which end becomes the mouth and which becomes the anus is conserved across deuterostomes. A protein signaling system involving beta-catenin (part of the Wnt pathway) is one of the earliest to act, establishing the basic head-to-tail axis. Nodal signaling, working through the same family of proteins as BMP, helps pattern the oral ectoderm and set up the future mouth region. BMP and its antagonist chordin then subdivide the embryo along the back-to-belly axis. These pathways work in roughly perpendicular directions, creating a coordinate system that tells cells where they are and what to become.
This signaling architecture is ancient. The same BMP-chordin axis that patterns a frog embryo also patterns a sea urchin and, in modified form, an insect. The key difference in deuterostomes is the spatial relationship between these signals and the gastrulation site, which channels the blastopore toward an anal fate and positions the mouth-forming signals at the opposite pole.